1. Field of the Invention
The present invention relates to a control apparatus and a method for controlling an electric rotating machine for vehicle, in which the electric rotating machine acts as a starter motor when an internal combustion engine is started and acts as a generator after the internal combustion engine has started.
2. Description of the Related Art
It is a recent trend that a system, in which internal combustion engine stops upon stopping the vehicle (i.e., idle stop), has been increasingly adopted in vehicles in consideration of reduction in carbon dioxide emission and improvement in fuel consumption. In those vehicles, number of starts of the internal combustion engine increases inevitably. It is therefore preferable to adopt a combined starter motor/charging generator in which the internal combustion engine and the electric rotating machine are combined at all times and less noise is produced as compared with a conventional starter motor operated by engagement of a pinion with a ring gear (flywheel starter gear). For example, the Japanese Patent Publication (unexamined) No. 261419/1994 discloses a hybrid car that runs with a motor driven by electric power supplied from a generator driven by an internal combustion engine, in which this generator functions as a starter motor.
In the art disclosed in this Japanese Patent Publication (unexamined) No. 261419/1994, the generator functions also as a starter motor, which saves space for installing a starter motor. When a vehicle-mounted battery supplies an electric power to the starter motor, the electric power is supplied from an inverter that supplies electric power to the vehicle driving motor, and it is therefore necessary to temporarily stop supplying electric power to the vehicle driving motor. In order to prevent a shock caused by such stopping and resuming of electric supply generated during running of the vehicle, the internal combustion engine is started by the starter motor only when change in load of the vehicle driving motor is small.
The foregoing art is peculiar to hybrid cars and is not necessary for vehicles other than the hybrid cars. However, in the foregoing case where the combined starter motor/charging generator (hereinafter referred to electric rotating machine) is used for the purpose of reducing noise and saving space, the electric rotating machine requires a torque and a rotating speed for starting the internal combustion engine. Accordingly, in order to obtain these motor characteristics within a limited space, the generator characteristics are sacrificed and it is not possible to obtain charging voltage for the vehicle-mounted battery when the electric rotating machine rotates at a low speed. Therefore, when the electric rotating machine is used as a charging generator, it is necessary to perform switching between inverter (power) generation mode in which step-up chopper control of the generated voltage is carried out by applying a phase-controlled alternating current from an inverter to the armature coil and alternator (power) generation mode in which a predetermined target voltage is obtained by applying a field current.
This switchover between the inverter generation mode and the alternator generation mode is carried out through a method as shown in the form of operation characteristics in FIGS. 5 and 6. FIG. 5 shows a case where the power generation mode is switched (shifted) from the inverter generation mode to the alternator generation mode, and FIG. 6 shows a case where the power generation mode is switched from the alternator generation mode to the inverter generation mode. Referring to FIG. 5, in the case where the power generation mode is switched from the inverter generation mode to the alternator generation mode, when the rotating speed of the electric rotating machine has reached a predetermined value, a command for changing the field current to 0 is given at t11 of time base, and the field current is changed to 0 by a time constant of a field coil as shown in the drawing. Subsequently, a torque command value becomes 0 at t12, and an inverter-driving signal to apply the alternating current becomes “off”. The field current is increased gradually from t13, and the switchover to the alternator generation mode completes at t14.
On the other hand, in the case where the power generation mode is switched from the alternator generation mode to the inverter generation mode, referring to FIG. 6, when the rotating speed of the electric rotating machine has reached a predetermined value, a command for changing the field current to 0 is given at t21 of the time base, and the field current is changed to 0 by the time constant of the field coil. Subsequently, the inverter-driving signal to apply the alternating current to an armature coil becomes “on” at t22 under the state of the torque command value being 0, thus application of the alternating current being started. The field becomes “on” again at t23. A torque command is given at t24, and the inverter controls the alternating current according to this torque command. The switchover to the inverter generation mode completes at t25.
In the conventional control system of above arrangement, when the power generation mode is switched as described above, the field current is interrupted and recovered after the switchover, and it is therefore possible to prevent troubles such as breakage of a circuit element due to abnormal voltage, an impossibility to apply the alternating current due to the voltage generated by the electric rotating machine. However, in the case of carrying out either of these two switchovers of the power generation mode at a predetermined rotating speed, when the driving condition comes into a state that engine speed of the internal combustion engine is held at a specific rotating speed, the switchover between one power generation mode and another takes place frequently. Thus, the field current is interrupted and power generation is interrupted undesirably every time the power generation mode is switched over. This makes the vehicle-mounted battery to loose its balance between charge and discharge resulting in lowering power source voltage. Moreover, the driving torque required by the electric rotating machine varies depending upon the power generation mode even if the electric load is the same. Consequently, the switchover of the power generation mode is accompanied by a change in engine speed of the internal combustion engine, eventually resulting in occurrence of a shock.